1. Field of the Invention
The present invention relates to a draft control device of a tractor and, more particularly, to a draft control device of a tractor, which automatically lifts a three-point hitch so as to decrease a traction load when the plowing depth of a work machine, which is connected to the three-point hitch at the rear side of a tractor, becomes excessively deep while working such that an overload is applied, and a tractor including the same.
2. Description of Related Art
In general, tractors have advantages in the structure and characteristics of providing a work machine to a rear hitch so as to carry out various work depending on the purpose. For example, it is possible to carry out various farm work, for example, clod breaking, disease and insect pest control, pumping work, threshing and the like, by attaching a work machine, for example, a rake, a rotavator, a plow, a harrow and the like, to the hitch.
Such a hitch is lifted or lowered according to the lever operation of a control device provided in the vicinity of the operator seat of a tractor such that the height of a work machine provided to the hitch is controlled. Therefore, when carrying out a target work in a state, in which a work machine is attached to the rear side of a tractor, a worker carry out appropriate work according to the kind of the work machine by appropriately operating the lever disposed in the vicinity of the operator seat.
In the case where plow work is to be carried out using a plow among the work machines attached to the rear side of a tractor, there is difficulty of varying degrees depending on the soil condition in maintaining desired plowing depth due to the characteristics of the plow since a plow blade is likely to grow in the soil during the plow work. Particularly, as the plowing depth is increased gradually, a work machine traction load is also increased, resulting in the generation of tractor slip and the increase of fuel consumption.
Accordingly, a draft control device is provided to the tractor so as to maintain a uniform traction load in active response to the traction load which changes during the plow work. This draft control device helps the smooth plow work without overload by automatically lifting the three-point hitch, when the plowing depth of the work machine, which is connected to the three-point hitch at the rear side of the tractor, becomes excessively deep, resulting in the overload.
FIG. 1 is a schematic diagram for illustrating a state, in which a plow is attached to a three-point hitch at the rear side of a tractor.
As shown in FIG. 1, during forward driving in a state, in which a plow 1 is attached to a three-point hitch, plow resistance is generated in the opposite direction of the driving direction. Further, as the plow work progresses, the plow 1 grows in the soil increasingly, resulting in the increase of the plow resistance. A lower link 2 is applied with an excessive traction load due to thus increased plow resistance and so the plow is rotated in the counterclockwise direction with respect to the hinge point 8 of the lower link 2.
The rotational motion of the plow 1 with respect to the hinge point 8 is transmitted to a top link bracket 4 through a top link 3 and, by the force, the top link bracket 4 is rotated in the clockwise direction relative to a top link bracket hinge point 9, wherein a traction load-sensing rod 6, which is connected to the top link bracket 4 is pushed in an arrow direction such that oil pressure corresponding to the displacement of the traction load-sensing rod 6 is compensated by a lifting cylinder through a lifting control valve (not shown) and thus the plow is lifted by a predetermined height.
FIG. 2 is a cross-sectional view of a prior art draft control device of FIG. 1, which includes the top link bracket.
Referring to FIG. 2, the top link bracket 4 has an upper end hinge-coupled to a cylinder body 5, which is integrally formed at the rear side of the mission case of the tractor, and a lower end for providing restoring force in a direction for pushing the top link 3, which is connected to the top link bracket 4, through a traction guide pin 11, which is elastically supported by a traction spring 10. Herein, the top link bracket 4 has a plurality of top link connection holes a, b, c, which are different in height.
In this prior art draft control device, the sensitivity of the draft control function is largely differed depending on the position of the top link connection holes a, b, c of the top link 3, at which the work machine (the plow 1) and the top link bracket 4 are connected to each other, that is, which one of the top link connection holes a, b, c of the top link 3 is used for connecting the work machine (the plow 1) and the top link bracket 4. In other words, the sensitivity of the draft control function (the draft function) can be adjusted by changing the use position of the holes of the top link bracket 4.
In the structure, in which the three connection holes are formed as shown in FIG. 2, if the top link 3 is connected to the upper hole a, the draft control function is not executed while, if the top link 3 is connected to the lower hole c, the top link bracket 4 sensitively reacts even to small force and thus the draft control function is executed. Of course, in the case of the middle hole b, the top link bracket 4 reacts less sensitively, compared with the connection to the lower hole c.
As described above, the prior art draft control device appropriately adjusts the sensitivity of the draft control device depending on the soil condition by connecting the top link to appropriate one of the plurality of connection holes. However, the prior art draft control device has disadvantages that a pin has to be inconveniently separated so as to change the position of the connection hole to be used whenever adjusting the sensitivity and more delicate sensitivity adjustment than intervals among the holes cannot be realized since the positions of the holes are fixed.